The present invention relates in general to an array device having functional circuits for a plurality of channels such as optical receiver modules for optical interconnection or like optical receiver amodules, and more particularly to an array device including current generating circuits which are capable of supplying the same current to the functional circuits for a plurality of channels.
In recent years, as the multi-media which handles a text, a voice and an image have been developed, the request for promoting the large capacity and the high speed transmission of data in information/communication equipment is increased and as a result promotion of the high frequency and increase of the number of connections become the serious problems in the electrical interconnections within the equipment and between the equipment.
In particular, all the severe system requests for reduction of the waveform distortion and the attenuation, the EMI measures, the ground isolation, and the low power consumption become difficult to be fulfilled by the electrical interconnections.
On the other hand, the success in the optical communication technology shows that for the signal transmission of the high speed, large capacity and long distance, the procedure that an electrical signal is temporarily converted into light and the resultant light signal is transmitted through a transmission line formed of optical fibers is more advantageous than the procedure that an electrical signal is directly transmitted as it is. That is, it is shown that the light transmission in which the electro-optic and opto-electric conversion function is introduced into a system is excellent in terms of practical use.
Therefore, while it is evident that the optical communication technology becomes more and more the important technology in the future, it is a urgent necessity to develop the optical interconnection technology for solving the above-mentioned problems associated with the electrical interconnection by utilizing the various features of light.
In addition, in recent years, the performance of element devices which are essential for realizing the optical interconnection has been drastically enhanced, and also the progress in the element technology development relating to the optical and electrical mounting results in a module in which the electrical connection can be replaced with the optical interconnect being actively developed. But, in the optical interconnect module aiming at replacement from the electrical interconnect, there are many requirement problems, which are considered to be essential for the practical use, such as "to realize good convenience similar to an interface IC (integrated circuit device)", "to be small size", and "to attain both low power consumption and low cost".
In particular, in the optical transmitter module out of the multi-channel parallel interconnect modules, there is employed an array of semiconductor lasers or light emitting diodes as the electro-optic conversion devices. Each of these devices is basically a current driven device and hence requires a constant current source a current from which is controlled at a constant DC value in order to determine the bias current and a constant amplitude of a pulse driving current for extracting the device characteristics. FIG. 1 is a circuit diagram showing a transmission driver as a main portion of the optical transmitter module. In the figure, the transmission driver has transistors Q1 and Q2, and to bases thereof are inputted inputs 1 and 2, respectively. A constant current circuit 100X is connected to each of emitters of the transistors Q1 and Q2. A load DIODE is connected to a collector of the transistor Q2. The load DIODE is also connected to a constant current circuit 100X' in order to secure a high speed operation in the transmission driver.
On the other hand, FIG. 2 is a circuit diagram showing a configuration of a receiver circuit as a main portion of an optical receiver module. The incident light signal is converted into a current signal I.sub.IN through a photodiode and an output is obtained from a transimpedance amplifier ZAMP. A constant current generating circuit 100Y is used to compensate for a dark current of the photodiode and also to apply an offset to the input current signal.
But, the array device constituting the optical interconnect module is configured, from the circumstances in which the characteristics of the individual original constituent elements are equal to each other, in such a way as to have only to be able to supply the same current to all the constant current generating circuits of the channels.
In this connection, in the conventional design, since the high speed parallel type integrated circuit has the configuration in which the power sources and the grounding points are provided independently for every circuit in many cases, the constant current generating circuits thereof are necessarily provided independently of one another.
In such cases, since the individual unit circuits constituting the array circuit are configured in such a way as to have the independent power sources, ground and control circuits, there is the merit in which the crosstalk between the channels through direct wiring path can be in principle readily suppressed.
On the other hand, however, since not only the number of external control signals to be supplied to the circuit and the number of input/output lines containing the power source lines become enormous, but also the individual channels need to be adjusted independently of one another during use of the modules, the labor for the adjustment is complicated and also the increase in the system cost is immeasurable.
In addition thereto, it is not said as the practical means that the individual ones of a plurality of wiring lines require the fine manual adjustment on the printed card board.
Therefore, it is desirable that the power sources and the ground lines are respectively made common so that the current to be caused to flow is controlled from one common terminal.
Referring to FIG. 3, there is shown an equivalent circuit diagram, as a prior art example, of a circuit which is designed in such a way that circuits of multiple channels are controlled at the same time by one common voltage. In addition, referring to FIG. 4, there is shown the outline of an arrangement of current generating circuit blocks 100A' (301', 201', and 101') to 100N' (30n', 20n' and 10n') of channels and functional circuit blocks 401' and 402' to 40n' which are formed on an integrated circuit chip 1'.
In this example, a control signal VCTRL which has been inputted to a control signal input terminal IN is caused to flow through the positions of the current generating control blocks 100A' (301', 201' and 101') to 100N' (30n', 20n' and 10n') of the channels which are arranged in the distributed positions on the integrated circuit chip so as to generate the desired currents therefrom, and also the circuit blocks 401' and 402' to 40n' to which the constant currents are supplied from the respective current generating circuit blocks 100A' to 100N' are also distributively arranged on the integrated circuit chip. Incidentally, reference numerals 301' and 302' to 30n' designate respectively input circuits each comprised of an active circuit or a low-pass filter.
While such a layout is conventionally general, with such a layout, the ground voltage level as the reference in each of the distributed positions changes every channel due to the finite resistance which the wiring on the chip has, and hence the current generating circuits are controlled on the basis of the substantially different voltages. Then, as a result, there arises a problem that the dispersion occurs in the currents which are generated for the respective channels.
On the other hand, in the high speed optical interconnect module, the semiconductor laser is employed in order to convert an electrical signal into, a light signal. In general, it is well known that the semiconductor laser device has the property in which the characteristics thereof are sensitive to the change in the temperature, and the influence of the temperature change appears in the form of increase of the lasing threshold and reduction in the efficiency of converting the current into light. Then, both the change in the optical output and the change in the frequency bandwidth resulting from the temperature change in the oscillation threshold and the conversion efficiency influence directly the signal transmission characteristics of the optical interconnect module.
Then, therefore, in the optical communication system, in order to suppress the temperature change, a control circuit is incorporated therein to be used which is designed in such a way as to monitor the intensity of a laser output beam so as for the laser output beam to maintain the fixed average intensity at all times. However, in the field of the optical interconnect in which the requirement for the miniaturization of the module is strict, it is difficult in reality to incorporate a large scale circuit for such control in order to use it.
For this reason, conventionally, the circuit system has been adopted such that with respect to the signal which is used to control the output current of FIG. 3, the input voltage is made constant and also the output current is kept constant as much as possible. As a result, however, there arises a problem that the signal transmission characteristics of the optical interconnect module are changed in such a way that: the dynamic range of an input of the receiver circuit becomes narrow; the error rate is changed along with the change in the ambient air temperature; and the band width is changed.
As described above, in the interconnection technology, in particular, in the transmitter/receiver circuit of the parallel optical interconnect module which is connected to a plurality of optical fibers in order to carry out transmitter/receiver of the optical signal, for example, the transmission functional circuit blocks for a plurality of channels constituting the parallel optical interconnect module requires the constant current source in which the current to be supplied can be controlled from the outside, and also it is essential thereto that all the channels can be controlled at the same time by one common voltage signal. In other words, while the semiconductor laser devices each of which serves to convert the electrical signal into the optical signal are respectively employed in the transmission functional blocks, the semiconductor laser device has the feature that its output is changed along with the temperature change and the current change.
Then, the transmission functional circuit blocks are provided so as to correspond to a plurality of channels, and hence in order to make the output adjustable, the constant current source is required in which the current value of the driving signal which is supplied to each of the transmission functional circuit blocks can be controlled from the outside. In addition thereto, in order to make the output adjustment able to be readily carried out, it is essential that all the channels can be controlled simultaneously by one common voltage signal.
However, there arises a problem that the above-mentioned conditions are not essentially fulfilled in the conventional circuit and even if those conditions are fulfilled in terms of circuit configuration, the circuit does not have the characteristics essential to the optical interconnect, or even if the circuit has such characteristics, the characteristics are insufficient.
In the light of the foregoing problems associated with the prior art, it is therefore an object of the present invention to provide an array device having functional circuits for a plurality of channels which is capable of preventing the ground offset voltage from influencing thereupon.
It is another object of the present invention to provide an array device having functional circuits for a plurality of channels which is capable of relatively reducing the crosstalk between the channels of the functional circuits.